This invention pertains to promoted skeletal iron catalysts for use in Fischer-Tropsch synthesis processes for converting CO and H2 in synthesis gas feeds to produce various desired hydrocarbon products. It pertains particularly to such promoted skeletal iron catalysts having specific metal compositions and which are prepared using improved method steps for providing improved performance including elevated middle-distillate yield, increased attrition resistance and catalyst settling rates for advantageous use in slurry-phase catalytic reactors.
The Fischer-Tropsch (F-T) catalytic synthesis process is a well-known and effective route for converting synthesis gas feeds containing CO and H2 into chemical feedstocks and hydrocarbon liquid fuels. Precipitated iron catalysts which have been under extensive development for such processes can achieve high catalytic activities, but break down easily to sub-micron particle size during reactor operations for production of hydrocarbon liquid fuels. Such particle breakdown results in difficulties for separating the very fine catalyst particles from the hydrocarbon liquid and waxy products, and has thus hindered commercialization of such F-T synthesis process. Some skeletal iron catalysts suitable for use in F-T synthesis reaction processes have been recently disclosed by U.S. Pat. Nos. 6,265,451 and 6,277,895 to Zhou et al. However, further improvements in such skeletal iron catalysts are desired.
The promoted skeletal iron catalysts of the present invention provide good potential for use in commercial F-T synthesis processes, particularly in slurry type reactors, because the catalyst has lower cost per unit mass of active metal, and has good resistance to particle attrition during vigorous hydrodynamic conditions existing in slurry phase reactors. Also, the used catalysts are readily separable from F-T waxy products by simple gravity sedimentation or by filtration of the catalyst.
This invention provides improved promoted skeletal iron catalysts containing mainly iron and promoted with minor amount of copper, manganese, and potassium. The promoted skeletal iron catalysts contain 50-90 wt % iron, 0-5.0 wt % copper, 0.1-5.0 wt % manganese, and 0.1-3.0 wt % potassium with the remainder being aluminum. The catalyst has good particle strength and provides high catalytic activity and selectivity towards the formation of desirable hydrocarbon products from the CO and H2 feedstreams, particularly liquid transportation fuels. The promoted skeletal iron catalyst has catalytic activity comparable to that of precipitated iron catalyst, and more favorable product selectivity towards diesel fuel than that of either precipitated or fused iron catalysts, and can be used in either fixed bed catalytic reactors or slurry bed type reactors for Fischer-Tropsch synthesis processes for producing desired hydrocarbon products. The resulting hydrocarbon liquid products are also rich in alpha-olefins which are valuable feedstocks for chemicals production.
The promoted skeletal iron catalysts of this invention are prepared utilizing improved method steps, which include mixing together iron chips or powder with the non-ferrous metal chips or powders aluminum, copper and manganese, heating and melting the metal chips mixture to form a molten metal alloy, then rapidly cooling the molten metal alloy to room temperature (15-20xc2x0 C.) such as by quenching in water, and then pulverizing the resulting solid metal alloy to provide fine precursor iron alloy particles having size range of 0.1-10 mm (10-10,000 microns). A major portion of the aluminum is removed by extracting/leaching by contacting with a suitable caustic solution such as NaOH or KOH to provide base iron catalyst particles. The base catalyst particles are next impregnated or loaded with the potassium promoter to provide 0.1-3.0 wt. % potassium. The skeletal iron catalyst is activated utilizing either high-temperature fixed-bed catalyst activation or in-situ activation method steps. The resulting promoted skeletal iron catalysts have high activity for catalyzing conversion of H2 and CO contained in syngas feedstreams to produce various desirable hydrocarbon products, and are attrition resistant and do not breakdown easily to undesired submicron particle size during extended reaction operations. Because the spent catalyst particles have higher density and are readily separable from the hydrocarbon product liquids by gravity sedimentation, this skeletal iron catalyst is especially suitable for use in slurry-phase type reactors for Fischer Tropsch synthesis processes.
The promoted skeletal iron catalysts of this invention provide various advantages compared with conventional precipitated iron or fused iron catalysts, and also provide specific improvements over the known skeletal iron catalysts. These advantages includes simplified catalyst preparation methods utilizing large size metal chips for forming a molten metal alloy which is cooled rapidly to room temperature and pulverized to desired particle size range to provide a catalyst precursor material, and an improved method for loading the potassium promotor into the catalyst precursor. The catalyst can be effectively activated in either fixed-bed or in-situ reactors. These promoted skeletal iron catalysts provide good syngas conversion (CO conversion greater than 80%) comparable to precipitated iron catalysts under industrial conditions, and have relatively stable activity and excellent selectivity for liquid transportation fuels and alpha-olefins, particularly in slurry-phase Fischer-Tropsch synthesis processes. Also, these catalysts provide good attrition resistance and sedimentation separation of used catalyst from liquid hydrocarbon slurries withdrawn from slurry-phase Fischer-Tropsch reactors.
The present invention provides improved promoted skeletal iron catalysts having unique composition for advantageous use in Fischer-Tropsch synthesis processes for CO and H2 feedstreams for producing desired hydrocarbon liquid products. The promoted skeletal iron catalysts contain 70-90 wt % iron with the remainder being less than 10 wt % non-ferrous metal promoters of copper and manganese, and 0.1-3 wt % potassium, with the balance being aluminum. The promoted skeletal iron catalysts have surface areas in the range of 20-80 m2/g, and preferably 30-65 m2/g, and particle size range of 10-10,000 microns.
The catalyst preparation method steps for this invention includes first mixing iron chips/powder together uniformly with selected non-ferrous metal chips/powder, particularly aluminum, copper, and manganese in the proportion of 20-80 wt. % iron and 30-70 wt. % non-ferrous metals. A typical mixture may contain by weight 35-55% iron, 40-60% aluminum, 1-15% manganese and 0-10% copper. The metals uniform mixture is heated and melted to form a molten metal alloy, which is cooled rapidly to room temperature (15-20xc2x0 C.) such as by quenching in water, and then pulverized to provide catalyst precursor particles having a 10-10,000 micron size range. The catalyst precursor particles are then extracted or leached by utilizing a suitable caustic solution such as 10-50% NaOH or KOH solution at 50-90xc2x0 C. temperature for sufficient time such as 20-150 minutes to remove a major portion of aluminum from the iron alloy, and thereby form the base skeletal iron catalyst precursor material.
The resulting base skeletal iron catalyst precursor is next promoted with potassium by impregnating the potassium onto the base skeletal iron catalyst utilizing a suitable organic alcohol solution containing potassium, and then dried to evaporate the alcohol solvent and provide the promoted skeletal iron catalyst containing 0.1-3 wt. % potassium. Suitable alcohol solutions containing potassium may include but are not limited to methanolic potassium hydroxide, ethanolic potassium hydroxide, or potassium carbonate.
The dried promoted skeletal iron catalyst is next reduced and/or activated such as by utilizing a hydrogen flow of 0.05-1.0 NL/g-cat/h in a fixed-bed reactor at a temperature of 300-350xc2x0 C. for 2-12 hours to provide an active promoted skeletal iron catalyst, which can be then mixed with a suitable reaction medium such as liquid paraffin or ethanol and transferred into a reactor for use in F-T synthesis processes. Alternatively, the promoted skeletal iron catalyst can be activated or reduced by mixing with a suitable reaction medium such as liquid paraffin to provide a slurry form, and then transferred directly into a slurry-phase F-T reactor and treated with hydrogen at a flow rate of 0.3-3.0 NL/g-cat/h and 300-350xc2x0 C. temperature for 3-48 hours. The skeletal iron catalyst is then ready for effective use in catalytic F-T synthesis processes utilizing CO and H2 feedstreams for producing desired hydrocarbon products.
The final promoted skeletal iron catalyst particle size can be within a 10-10,000 micron range, with the larger particle size range of 1000-10,000 micron being for use in fixed bed reactors, and the smaller particle size range being 20-200 micron for use in slurry-phase reactors.
1. Preparation of Catalyst Precursor Particles
Mix together uniformly chips/powders of iron and non-ferrous metals selected from aluminum, copper and manganese to provide an iron content of 35-55 wt. %, 40-60 wt % non-ferrous metals and 5-15 wt. % manganese in the form of a carbonate compound. Heat and melt the uniformly mixed metals chips/powders in a suitable furnace such as an electric arc induction furnace to provide a molten metal alloy, and cool the molten alloy rapidly to room temperature (15-20xc2x0 C.) by quenching in a suitable liquid such as water. Then mechanically pulverize the resulting solid metal alloy to provide iron alloy catalyst precursor particles having particle size range of 10-10,000 micron.
2. Preparation of Base Skeletal Iron Catalyst
The skeletal iron catalysts are prepared from the iron alloy catalyst precursor particles under an inert gas blanket by the following procedure:
Add a sufficient volume of caustic NaOH or KOH solution (10-50% concentration) into a stirred container, heat the solution to a temperature of 30-95xc2x0 C., add the iron alloy particles (10-10,000 micron size) into the caustic solution, maintain the reaction condition for 2-150 minutes after the iron alloy particle addition is complete, and extract and/or leach out a major portion of the aluminum from the iron alloy particles. Then wash the treated iron alloy particles with deionized water to pH=7, replace the water with water-free ethanol, and temporarily store the resulting base skeletal iron catalyst particles in ethanol.
3. Impregnation of Potassium Promoter on the Base Skeletal Iron Catalyst
The impregnation of potassium promotor onto the base skeletal iron catalyst from step 2 is performed as follows:
Prepare an alcohol solution containing a potassium compound such as a carbonate or nitrate having the desired potassium content based on the iron content in the catalyst, to provide a potassium to iron weight ratio of 0.5-3:100. Transfer the base catalyst particles with alcohol solution to an rotating evaporator, then pour the required potassium solution into the same evaporator vessel, and vaporize the alcohol under controlled conditions of 40-80xc2x0 C. temperature and 100-500 mm Hg vacuum pressure.
4. Activation of Promoted Skeletal Iron Catalyst
The promoted skeletal iron catalyst particles, after drying under vacuum, can be activated in either of the following ways before being used in an F-T reactor.
The dry catalyst particles are transferred into a fixed-bed activation reactor for activation with hydrogen, and then transferred to a F-T reactor, under inert gas protection while being transferred, the catalyst is then activated under a stream of hydrogen (0.05-1.0 NL/g-cat/hr) at a temperature of 300-350xc2x0 for 2-12 hours.
After drying the impregnated catalyst particles, add sufficient liquid reaction medium such as liquid paraffin into the vessel to form a slurry, which is then transferred directly into the F-T reactor for in-site activation using hydrogen (0.3-3.0 NL/g-cat/hr) at 300-350xc2x0 for 3-48 hours.
The promoted skeletal iron catalysts of this invention are uniquely useful in catalytic processes for Fischer-Tropsch synthesis of CO and H2 feedstreams, particularly in slurry-phase reactors, to produce desired hydrocarbon products. Useful reaction conditions are H2/CO molar ratio of 0.5-5:1 in the feedstreams and 5-40 wt. % catalyst loading relative to a reaction medium such as liquid paraffin, catalyst particle size of 1-10 mm (1000-10,000 microns) for fixed-bed reactors and 20-200 microns for slurry-phase reactors, 200-350xc2x0 C. reaction temperature, 1.0-3.0 MPa system pressure, and gas hourly space velocity of 0.5-5 L/g-cat/h. A desired hydrocarbon liquid product containing used fine catalyst particles is withdrawn, and the fine catalyst particles are substantially removed from the liquid by sedimentation settling of the particles.
The improved preparation methods for the promoted skeletal iron catalyst of this invention and its performance are further disclosed by the following examples, which should not be construed as limiting the scope of the invention.